When using the Active Topics feature, the forums and topics listed on the screen are dependent on the date & time that you last logged in on MLS. The default setting for the screen Interval setting (i.e. date range) is based on your last Login/Logout time-stamp.
So, for example if no new activity has taken place on the DCC forum since you last logged in it won't show up on the Active Topics screen.
If nothing is listed for a particular forum that you're interested in the easiest way to see what's been going on is to widen the date range by changing the Interval setting to, Yesterday, Last Week, Last Month, etc. Until you get the the most recent activity in that forum listed on the Active Topics screen.
If at the widest date range setting there is still nothing listed (highly unlikely), you could use the All Forums screen, accessed via MLS menu-bar > Community menu > Forums path, to directly access a listing of all forums available, and then click the link for the forum that you're interested in.